1. Field of the Invention
The present invention concerns a novel canister assembly for a cast booster explosive, and cast booster explosives comprising the canister assembly. The cast booster explosives are of the type utilized to detonate normally cap-insensitive explosives such as ammonium nitrate and fuel oil (“ANFO”) explosives. The present invention also concerns an efficient method of making the canister assembly by separate injection molding of the canister and its cap well, and for making a cast booster explosive.
2. Related Art
Cast booster explosives are normally cast with a fuse tunnel and a cap well, the tunnel providing a passageway for a fuse connected to a detonator, sometimes referred to as a detonator cap or simply cap, which is received within the cap well. This construction is usually attained by pouring a flowable explosive into a booster shell, i.e., a canister, having placed within it a removable molding fixture which forms within the cast explosive a fuse tunnel open at both ends and one or more cap wells open to the exterior of the canister at one end and closed at its other end. The flowable explosive is caused or allowed to harden or set within the canister to provide the cast booster explosive, and the molding fixture is removed. It is known to insert into the open end of the cap wells, prior to complete hardening of the explosive, a disc-like locking member to retain a detonator within the cap well.
The prior art also shows canisters for cast booster explosives having fuse tunnels and cap wells as part of a canister body into which a flowable explosive is placed to cure or harden. For example, U.S. Pat. No. 4,334,476 issued Jun. 15, 1982 to John T. Day et al. for “Primer Cup” discloses a canister or cup having a central fuse conduit 18′ (FIG. 1) open at each end and a conduit 26 closed at its top end 28 which serves as a cap well. The cup 10 may be made of polyethylene (col. 5, lines 30-31) or other resins by an injection molding process (col. 8, lines 57-68). In addition see U.S. Pat. No. 3,183,836 issued May 18, 1965 to G. L. Griffith for “Canister For Cast Primer”, which discloses a two-part plastic (col. 1, lines 13-25) container 11 and cover 12. See FIG. 5 and col. 2, lines 21-27. Integral ribbed wells 20 and 21 serve as, respectively, a fuse tunnel and cap well.
U.S. Pat. No. 3,955,504 issued May 11, 1976 to Russell H. Romney for “Explosive Booster Casing” shows two hemispherical halves having, when fitted together (FIG. 4), a cap or cord-receiving tube 14 and a cap or cord-receiving tube 27. See col. 3, lines 33-58.
U.S. Pat. No. 3,407,730 was issued on Oct. 29, 1968 to G. L. Griffith for “Retainer For Holding A Detonator In A Detonator Receptacle And Explosive Cartridge Container Containing The Same”. FIGS. 1 and 2 thereof show a “digitated retainer” as an integral portion of an end cap made of a plastic material such as polypropylene which is molded in one piece with a central opening 2 into which six fingers 3 extend, as described at col. 4, lines 1-35.
U.S. Pat. No. 6,112,666, “Explosives Booster and Primer”, issued on Sep. 5, 2000 to C. M. Murray et al. shows a booster explosive having an integral fastening means for retaining in place non-electric tubing (shock tube) serving as the fuse of the detonator retained within the booster.
In use, cast booster explosives are normally lowered into a borehole, which may be as deep as 10, 20 or 30 feet (3, 6.1 and 9.1 meters, respectively) or deeper. More than one booster may be loaded into a given borehole and in such case the two or more boosters are normally positioned at different depths within a given borehole. The booster explosive(s) are employed to initiate a bulk explosive such as an ANFO (ammonium nitratefuel oil) slurry or emulsion which is poured into the borehole.
In any case, lowering the booster explosive into the hole may be done by means of the fuse which extends from the detonator through the booster explosive and to the surface for connection to a trunk line. This can put considerable stress on the fuse, sometimes resulting in the cap being dislodged at least partially from the cap well, which presents the danger of a malfunction. Even when a separate downline, such as a low energy detonating cord is used to lower one or more booster explosives into a borehole, stress may be placed on the detonator fuse ends by handling and lower the booster explosives.